1. Field of the Invention
The invention relates to a semiconductor device, particularly, a thin film transistor disposed in a display pixel of a display device.
2. Description of the Related Art
In general, a liquid crystal display device of an active matrix type is formed with a pixel selection thin film transistor in each of a plurality of display pixels arrayed in a matrix on an insulation substrate such as a glass substrate.
FIG. 8 is an equivalent circuit diagram of a display pixel of a liquid crystal display device. FIG. 8 shows one of a plurality of display pixels disposed on an insulation substrate. As shown in FIG. 8, a gate wiring GL to which a pixel selection signal is supplied from a vertical driving circuit DR1 and a drain wiring DL to which a display signal is supplied from a horizontal driving circuit DR2 are crossing each other.
A pixel selection thin film transistor (hereafter, referred to as a “thin film transistor”) TR is disposed in a region surrounded by the gate wiring GL and the drain wiring DL. A gate of the thin film transistor TR is connected to the gate wiring GL, and a drain thereof is connected to the drain wiring DL. A source of the thin film transistor TR is connected to a storage capacitor Cs storing a display signal and a pixel electrode as one of a pair of pixel electrodes interposing a liquid crystal LC therebetween. Another electrode of the pair of electrodes interposing the liquid crystal LC therebetween is connected to a common potential Vcom.
Next, the summary of the structure of the above-described display pixel will be described although not shown in the figure. A semiconductor layer made of, for example, a polysilicon layer is formed in a plurality of regions on an insulation substrate with a buffer film as an insulation film, for example, a silicon oxide film or a silicon nitride film interposed therebetween. The source and the drain are formed in the semiconductor layer by adding an impurity thereto, the semiconductor layer being an active layer of the above-described thin film transistor. A channel is formed between the source and the drain. Another semiconductor layer forming the above-described storage capacitor is formed adjacent to this semiconductor layer or at a distance from this semiconductor layer.
A gate insulation film made of, for example, a silicon oxide film or a silicon nitride film is formed on the buffer film, covering the semiconductor layer. A gate wiring made of, for example, chromium or molybdenum is formed on the gate insulation film, being partially opposed to the semiconductor layer. The gate wiring on the semiconductor layer that is the active layer is a gate electrode of the thin film transistor, and another semiconductor layer forms the storage capacitor together with the gate insulation film and the gate wiring. An interlayer insulation film made of, for example, a silicon oxide film or a silicon nitride film is formed on the gate insulation film, covering the gate wiring.
Furthermore, the interlayer insulation film on the source and drain of the semiconductor layer is provided with contact holes. A source wiring and a drain wiring are formed on the interlayer insulation film, being connected to the source and the drain through the contact holes respectively.
Furthermore, a planarization film is formed on the interlayer insulation film, covering the source wiring and the drain wiring and having an opening in a position on a part of the source wiring. A pixel electrode is formed on the planarization film, being connected to the source wiring through the opening. A first alignment film is formed on the pixel electrode. A common substrate formed with a second alignment film and a common electrode is disposed being opposed to the pixel electrode. A liquid crystal is sealed between the first alignment film and the second alignment film. The common electrode is connected to a common potential Vcom.
The relevant technology is disclosed in Japanese Patent Application Publication No. 2005-117069.
In the thin film transistor of the conventional art, however, external light, display light from the display pixel, or reflected light of these enters a depletion region of the semiconductor layer to generate an electron-hole pair and cause a light leak current, thereby degrading a display quality. The depletion region of the semiconductor layer means a region formed between the source or drain and the channel of the thin film transistor, where carriers do not exist.